In many technical fields of application, the need exists to measure surfaces of objects and thus also the objects themselves with high accuracy. This applies in particular to the manufacturing industry, for which the measuring and checking of surfaces of workpieces has great significance, in particular also for the purposes of quality control.
Coordinate measuring machines are typically used for these applications, which enable a precise measurement of the geometry of an object surface, typically with micrometer accuracy. Objects to be measured can be, for example, engine blocks, transmissions, and tools. Known coordinate measuring machines measure the surface by establishing a mechanical contact and scanning the surface. Examples thereof are gantry measuring machines as described, for example, in DE 43 25 337 or DE 43 25 347. Another system is based on the use of an articulated arm, the measuring sensor of which, which is arranged at the end of the multipart arm, can be moved along the surface. Articulated arms of the type in question are described, for example, in U.S. Pat. No. 5,402,582 or EP 1 474 650.
In the prior art, an optical or tactile sensor is used as a standard measuring sensor with such coordinate measuring devices, said tactile sensor consisting, for example, of a ruby sphere which is mounted on a measuring rod. The deflection of the tactile sensor, in the case of a coordinate measuring machine designed for three-dimensional measurements in three directions X, Y, and Z perpendicular to one another, is determined during the scanning via a switch element or distance-measuring element. The location of the contact and therefore the surface coordinates are calculated on the basis of the switching point or deflection distance.
To reconstruct the surface profile from the measurement data, the mechanical dimensions of the sensor itself and its alignment during the contact with the object surface must be considered. The sensor is designed having a measuring tip of known geometry, typically spherical or, for special applications, ellipsoidal, typically having a (main) radius in the order of magnitude of several millimeters. The term “measuring tip” is to be understood in conjunction with the present invention in general as a (tactile) measuring sensor of any arbitrary shape and dimensions, wherein it does not necessarily have to (but can) have a shape tapering to a point. The raw data measured using the coordinate measuring machine while using a tactile sensor represent the measured location coordinates of a reference point of the measuring tip, for example, the measuring tip center.
However, the measurement resolution is restricted because of the physical dimensions of the measuring tip of the tactile sensor. The physical dimensions of the measuring tip and/or the limited measurement resolution linked thereto result in a “smoothing effect” during the measurement of rough surfaces: While protrusions or peaks of an object surface can be measured nearly perfectly or faithfully to the object, the measuring tip of the tactile sensor cannot penetrate into narrow depressions of an object surface because of its physical dimensions. This causes a smoothing of the measured surface profile in a nonlinear manner, by the measurement data of depressed surface regions being smoothed while the measurement data of raised surface regions are nearly faithful to the object. For engineering aspects, this is even often advantageous, because, in particular for a planar connection of surfaces of two objects, an accurate knowledge of the raised regions thereof is more important than the accurate determination of narrow depressed surface regions.
On the other hand, the resolution of tactile measurements, in particular for a more accurate measurement of surface depressions, is no longer sufficient for many new applications because of the above mentioned limitations inherent to the method.
Therefore, approaches have been followed in the meantime in the prior art for contactless measurement, in particular using optical sensors. Surface depressions can also be measured very accurately by means of an optical sensor using an emitted measuring light beam, in particular from a laser, as long as the focus of the measuring light beam, in comparison to the measuring tip of a tactile sensor, on the object surface is not larger than the structure of its depressions. The resolution of optical measuring methods can accordingly be significantly higher than that of tactile measuring methods for accurate measurement of surface profiles, in particular the depressions thereof. Accordingly, a profile prepared using an optical sensor differs from a profile prepared using a tactile sensor of the same object surface. A surface profile prepared using an optical sensor, like a surface profile prepared using a tactile sensor, also represents a depiction of the actual object surface filtered in its resolution and based on the physical dimensions of the “measuring tip”, wherein the dimensions of the optical “measuring tip” can be considered to be converging toward the wavelength of the optical measuring radiation in comparison to the measuring tip of a tactile sensor or are negligible. Therefore, optical sensors and measuring methods for a coordinate measuring machine are suitable in principle for providing an actually object-faithful measurement of a surface profile.
Optical sensors or measuring methods for a coordinate measuring machine are linked to an array of advantages: The measurement takes place in a contactless manner, and the optical sensor can be guided more rapidly than a tactile sensor over an object surface, with smaller physical dimensions of the “measuring tip”, whereby a higher lateral resolution of the measurement is enabled.
Nonetheless, surface profiles prepared not only with tactile sensors, but rather also with optical sensors still always contain features which do not originate from the measured surface but rather are caused by the measuring method. For example, measurement errors in the height determination of a surface because of vibrations of the coordinate measuring machine used and method measures for suppressing these effects are known from DE 197 35 975.
The measurement results of optical sensors, in particular for interferometric measuring methods, are often disadvantageously influenced by phase noise or speckle effects. Depending on the roughness of the object surface, for example, the phase of the light reflected from a surface can be changed in such a manner that a distance measured to a targeted object point is incorrect. As a consequence of such local optical interfering influences, surface profiles measured using optical sensors often have measurement errors, for example, virtual singular peaks or depressions which do not exist in the object surface, however.
Presently stationary, tactile measuring devices are predominantly used for roughness measurement. The test subject is placed in this case on a measurement table and a needle is guided linearly over the object while in contact with the surface. In this case, height changes of the needle can be registered and a height profile can be derived therefrom. As mentioned above, the measurement resolution is also dependent here on the needle geometry.
One disadvantage of such a device is the requirement that the workpiece has to be placed on the measurement table for this purpose. Depending on the workpiece size and shape, this can be very complex and time-consuming. More recent approaches therefore propose the combination of a specific roughness sensor with a coordinate measuring machine, to be able to avoid a transportation of the test subject.
The “BMT MiniProfiler” from Breitmeier Messtechnik GmbH is known as such a sensor, for example. The sensor can be provided as an interchangeable system in a magazine for repeated coupling and decoupling to and from a CMM. The control and evaluation can take place directly in the CMM operator interface. The sensor is brought into contact with the object surface. The CMM remains stationary during the following roughness measurement, i.e., the sensor as a whole is not moved in relation to the object. The needle of the sensor is guided in this state over the surface. Vibrations caused by the CMM can be reduced in this way.
Nonetheless, remaining external vibration on the part of the CMM or position errors caused by other environmental influences cannot be precluded and can negatively influence the measurement result. Furthermore, nonlinear effects can occur during the movement of the needle (movement of masses), which cannot be taken into consideration by the sensor.
A further disadvantage of this roughness acquisition and the workpiece to be checked is the mechanical contact existing in this case between sensor and workpiece. Undesired changes of the surface can occur in this way, in particular in the case of sensitive workpiece surfaces, for example, special coatings.